1. Field of the Invention
The present invention relates to a particle detection system, and to a lithographic apparatus provided with such a particle detection system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The imaging of the pattern including small structures is very sensitive to dust and other contamination of the patterning device, possibly protected by a pellicle, and the substrate. Therefore, before imaging, the patterning device (and/or the pellicle protecting the small structures thereof) and substrate are tested for contamination, in particular for particles. If a particle is detected on the patterning device or on the substrate, the particle may be accepted (thereby accepting a fault area on the substrate), or it may be removed, or the patterning device or substrate may be rejected.
In conventional lithographic apparatus, a particle detection system directs a beam of radiation, in particular (but not necessarily) monochrome radiation, i.e. radiation having substantially one wavelength, on a surface of an object, for example, but not limited to, the patterning device or the substrate. The object and/or the beam move in order to scan the surface of the object. When the beam of radiation engages the surface of the object, the radiation is partially reflected according to physical laws of reflection (an exit angle is identical to an angle of incidence with respect to a fictitious line perpendicular to the surface (the normal)). Another part of the incident radiation may enter the object, such as the patterning device or substrate, and is refracted (according to Snell's law). In both cases, the beam is anisotropically redirected. When the beam of radiation engages a contaminating particle, the radiation is scattered, i.e. reflected isotropically.
A radiation detector is positioned with respect to the surface and the beam of radiation such that radiation reflected on the surface is not incident on the detector, but a part of the radiation scattered, i.e. being reflected in substantially every direction, by a particle or other contamination is incident on the detector. Thus, the detector receives radiation only when the beam of radiation is scattered by a particle or other contamination. The beam of radiation towards the surface (hereinafter: the illumination beam) and the beam of radiation received by the radiation detector (hereinafter: the detection beam) follow separate paths in the radiation detection device to prevent crosstalk between the two paths. This is necessary because the illumination beam intensity usually is many orders of magnitude higher than the detection beam intensity.
Because of the limited space available, a particle detection system according to the prior art has optics which are folded to fit the particle detection system into the available space. A rotating faceted polygon is used for scanning the surface of an object along a scan line, while the object moves parallel to the plane of the surface in a direction essentially at right angles to the scan line. In the path of the illumination beam, optical components like one or more mirrors and one or more lenses, such as a scan lens, are used to produce an almost telecentric illumination of the surface of the object. In the path of the detection beam, optical components like one or more mirrors and one or more lenses, such as a cylinder lens for focusing the detection beam on or near the polygon for reduction of beam size, and another cylinder lens near the detector for making the detection beam stigmatic, are used to produce light on a detector when a particle is present on the surface of the object in the illumination beam. The illumination beam and the detection beam may use the same facet of the polygon, though at different places. Accordingly, although the illumination beam produces a spot on the surface of the object moving along the scan line, the detection beam is static after reflection on the polygon facet. As a result, a small detector can be used for the detection of particles on the scan line. The detector detects an amount of light scattered by a particle, which amount is processed in calibrated detection circuitry to produce a signal indicating the presence of a particle or not, and to give an indication of the particle size. Since the detector just collects light, the performance of the particle detection system is independent of a spot size on the detector.
A part of the radiation incident on the surface of the object enters the object and is refracted, as above mentioned. Inside the object, the beam may be refracted and/or diffracted by the pattern and/or reflected one or more times. Depending on a number of parameters, such as the material, the size, the geometry, and the like, a part of the radiation that entered the object will leave the object again in the direction of the detector. In that case, the detector detects radiation not being scattered by a particle. As a result a detection circuit receiving a signal from the detector determines that a particle is present, although no particle is actually present. Such a detected, but not actually present particle will hereinafter be referred to as a ghost particle.
Due to the continuing increase in density of the patterns on the objects as used in the semiconductor component manufacturing industry, such as a reticle, the occurrence of ghost particles becomes more probable, and presents a new and growing problem. Thus, there is a need for a particle detection device that is able to discriminate accurately between physically present particles and ghost particles.
The optical components used in the particle detection system are to be cost effective, meaning that the desired function must be performed while at the same time accepting optical aberrations to a degree which does not impair the functionality of the system. These aberrations result in a certain detection spot size on the detector. Besides that, in combination with an asymmetry of the optical layout that is desirable for a compactness of the particle detection system, the aberrations cause the detection spot to move slightly over the detector in a scanning process. This detection spot movement adds to the spot size on the detector, i.e. the used area of the detector, and thus undesirably makes the particle detection system more sensitive to ghost particles (since the ghost discrimination is based on spatial filtering requiring a small spot size).